Effects of Intermolecular Hydrogen-Bonding Interactions on the Amide I Mode of N-Methylacetamide: Matrix-Isolation Infrared Studies and ab Initio Molecular Orbital Calculations

Abstract

Effects of intermolecular hydrogen-bonding interactions on the amide I mode of N-methylacetamide (NMA) are studied by matrix-isolation infrared (IR) spectroscopy and ab initio molecular orbital calculations. The wavenumbers of the amide I IR bands of NMA in Ar and N2 matrixes with various NMA/matrix gas mixing ratios are compared with those calculated for the monomer, dimers, and trimers of trans-NMA and the monomer and dimer of cis-NMA. The band at 1708 (1706) cm-1 in Ar (N2) matrixes is assigned to the amide I mode of the monomer of trans-NMA. The band observed at 1686 (1681) cm-1 in Ar (N2) matrixes with the NMA/matrix gas mixing ratio larger than 1/500 is assigned to the amide I band due to the dimers of trans-NMA. The bands observed at lower wavenumbers for samples with the NMA/matrix gas mixing ratio as large as 1/100 are assigned to the amide I bands of the trimers and larger clusters of trans-NMA. It is likely that the band observed at 1695 (1693) cm-1 in the Ar (N2) matrix arises from the dimer of cis-NMA, which is as stable as the trans-NMA dimers because of the formation of two hydrogen bonds in a cyclic form. Although there are two amide I modes in an NMA dimer and three in an NMA trimer, only one mode is strongly IR active in each species. The intrinsic amide I wavenumbers of individual peptide groups in NMA clusters, i.e., the amide I wavenumbers in the case where there is no resonant vibrational coupling between the peptide groups, are examined by calculating the amide I wavenumbers for the dimers and trimers whose constituent molecules other than the target molecule have the CO group(s) substituted with 13C and 18O. It is shown that the shifts of the amide I band to lower wavenumbers induced by hydrogen bonding to the CO group are 20−25 cm-1, while those induced by hydrogen bonding to the N−H group are 15−20 cm-1. These shifts are approximately in line with the changes in the CO bond lengths and are approximately additive if both the CO and N−H groups of a peptide group are hydrogen bonded.